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Here's how I would do it:
CREATE TABLE twoletters
( code CHAR(2) NOT NULL
, CONSTRAINT twoletters
CHECK ( CHARACTER_LENGTH(code) = 2
AND SUBSTRING(code FROM 1 FOR 1)
BETWEEN 'A' AND 'Z'
AND SUBSTRING(code FROM 2 FOR 1)
BETWEEN 'A' AND 'Z' )
);
Notice that I chose CHAR(2). This would allow the CHARACTER_LENGTH
condition to be omitted, but I left it in if you cannot change your
column from VARCHAR.
You may need to change the standard SQL functions
(CHARACTER_LENGTH and SUBSTRING) to match your specific database system.
For instance, in SQL Server you would use LEN(code) and
SUBSTRING(code,1,1).
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